1. Field of the Invention
The present invention is directed to improved tool joints used in wellbore operations; such tool joints that have wear-reducing material applied thereto, including, but not limited to hardfacings and carbides; and methods for applying such materials to tool joints.
2. Description of Related Art
“Thermal spraying” refers to a variety of processes for depositing both metallic and non-metallic materials on a substrate to form a coating. Metals, cermets, ceramics, plastics and mixtures thereof in the form of powders, rods or wires may be used as coating material. Heat for melting the material is supplied by electric arc, plasma arc, or combustible fuel gases and compressed air or process gases form an accelerated stream of molten coating material. The material builds up on the substrate and cools to form the coating.
Electric arc spray processes use electrically charged wire which is fed by a wire feeder to an arc spray gun in which the wires converge, arc, and melt in a high temperature zone (e.g. 15,000 degrees F. or higher) created by the arc. A compressed air stream is directed to the arc zone and atomizes the molten material produced from the melting wire. The stream flows from the gun for coating onto a desired substrate.
Molten particle velocities average 100 meters per second; deposit thicknesses average 0.001 to0.003 inches per pass; and deposition rates range between 10 to 40 pounds of material per hour depending on the material and the amperage. By compassing the arc in a gun head relatively little heat is transferred by the molten material to the substrate. Sprayable materials include, but are not limited to, carbon steels, stainless steels, oxides, carbides, nickel alloys, copper, copper alloys, bronze, aluminum, aluminum alloys, zinc, babbitt, and molybdenum. Such materials may be spray to produce a coating or to rebuild a part.
In certain known drilling operations in drilling for oil and gas, a drill string of pipe is used that is secured together by tool joints on which are rings of hardfacing applied for abrasion resistance. Drill collars and a drill bit are connected to the insertion end of the string. A “cased wellbore” is one in which a casing having an inside diameter adequate for passage of the drill string with tool joints, collars and bit is put in place. The rotating action of drilling causes severe wear on the tool joints. One prior art response to this wear problem was oxyacetylene welding of tubular rods filled with tungsten carbide, chromium carbides and/or chromium carbide formers (chromium and carbon) to apply hardbanding to the tool joints. The disadvantages of this method included slow welding speeds and extreme weld heat affected zones.
Tool joints are the connecting members between sections of drill pipe used in a wellbore drilling operation. One member (the box) has an internal thread and the mating member (the pin) has an external thread, by which means they are assembled into a continuous unit with the drill pipe, thus forming a drill string. These tool joints are larger in diameter than the pipes they connect. As drilling proceeds, the tool joints rub against the drilled hole and/or against the drilled hole lining (“casing”). The strength of the connection is engineered around the wall thickness and heat-treated properties of the box above the thread. In the drilling process the wall thickness above the thread thins as it rubs against the formation. The life of the drill pipe is predicated upon the remaining strength of the tool joint. Therefore, thinning of the tool joint material above the thread is undesirable.
One current prior art practice is to weld a cladding on the exterior diameter of the tool joint. The cladding is a material that will resist abrasion caused by the rubbing action against the earthen wall of the hole or the steel casing. In one prior art method tungsten carbide is used in a matrix of steel. As the hole drilling progresses, the procedure is to “case,” or insert, a smaller bore steel tube into the hole to drill deeper into the ground. The tungsten carbide wears against the casing and thins the wall, inviting rupture due to external or internal pressure. With this in mind drill pipe users have begun to clad the exterior of the tool joint with materials that are less abrasive to the casing but also less resistant to wear by the earth formation.
The advent of the electric arc in the late 1950′s and early 1960′s allowed the development of prior art hardfacing for tool joints with electric rods, either solid or tubular, filled with carbides or carbide formers. The Hughes Tool Company and Reed Roller Bit Company developed coatings that would provide good wear results at an economical price. Both companies produced tungsten carbide coatings. Reed Roller Bit Company used Gas Tungsten Arc Welding (GTAW), a process wherein an arc melted the base metal and then carbides were dropped into the molten puddle created by the arc without using external filler metal; the matrix was the tool joint base material. Reed roller Bit Company then developed a Plasma Transferred Arc (“PTA”) process. The PTA melted a steel or nickel base powder and deposited tungsten carbide into the molten puddle. The major problem with PTA was consistency of deposit to obtain a uniform bond.
The Hughes Tool Company developed one prior art process that used (the still current) Gas Metal Arc Welding (GMAW), melting a wire as a matrix and depositing carbide into the molten puddle. Both companies used cast-crushed tungsten carbide, which is hard and angular-shaped, but also brittle. The wear pattern of this material often showed the carbides worn flush with the matrix.
Almost simultaneously both companies developed the sintered tungsten carbide pellet. This pellet is a potato-shaped particle of tungsten carbide bound together with a binder of cobalt (see, e.g., U.S. Pat. No. 4,228,339). The wear pattern changed from a smooth, worn surface to one wherein the carbides protruded as the matrix wore away, leaving the more shock-resistant carbides protruding from the surface. U.S. Pat. No. 4,243,727 discloses sintered tungsten carbide granules embedded in an alloy steel matrix. Both the cast tungsten carbide and the sintered tungsten carbide pellets were relatively expensive. In order to trim costs, crushed sintered tungsten carbide made from crushed steel cutting tools was used instead of the pellets; but often the use of this substitute resulted in cut casing.
Another prior art approach was to apply carbides of smaller size. Carbides are measured in openings per inch of a screen. Certain standard carbides are approximately −16+30 mesh material, which will pass through a screen with an opening of 0.046 inches (1.18 mm). It will remain on the surface of a screen with the opening of 0.0234″ (0.6 mm). Fine tungsten is −60 mesh (0.009 inches or 0.250 mm) and +80 mesh (0.007 inches or 0.180 mm). The fine tungsten was relatively difficult to apply. Depending upon the exact location at which the welder (operator) introduced the carbide, it either floated on the molten steel pool or was all melted by the arc. Another problem was the density of the carbides. Cast tungsten carbide was heavier and thus more costly by volume. The sintered pellet was difficult to manufacture in this size. It was much lighter in weight and lower in melting point, and it was almost impossible to inject into the molten pool. The crushed sintered material in the fine particle size has sharp points susceptible to melting and alloying.
Quality control and consistency of deposit were also problems. The volume of carbide in these deposits was difficult, if not impossible, to monitor. The systems used to dispense the carbide in many cases were not accurate, and a hardbander could control his cost by simply cutting back on the amount of material he injected into the molten pool. Thus the drilling contractor has had varying results with tool joint hardbanding, particularly since the earth formations vary from location to location.
Another prior art development was the use of chromium bearing materials that are called “casing friendly” because they lower the coefficient of friction between their surface and the surface of the casing (see, e.g., U.S. Pat. No. 5,224,559 issued Jul. 6, 1993). Many of these products are less effective in the protection of a tool joint. In many cases the casing friendly materials were inlayed flush with the diameter of the tool joint, which did not provide maximum protection of a connection. With the casing friendly wires, however, an applicator could not alter or tamper with the alloy content of the wire. In general, the alloy content of the wire dictates its performance in lowering the coefficient of friction and controlling open hole wear of the tool joint. Higher alloy content generally improves both conditions, but often introduces the problem of cracking.
In the prior art a variety of methods have been used to apply wear-reducing material to tool joints: GMAW (gas metal arc welding), GTAW (gas tungsten arc welding), PTA (plasma transferred arc), and FCAW (flux cored arc welding). These welding processes are characterized by establishing an arc between an electrode (either consumable or non-consumable) and tool joint base material. Once this arc is established, intense heat forms a plasma. The gas that forms the plasma is furnished by means of an external gas or an ingredient from a tubular wire. The temperature of the plasma is in excess of 10,000 degrees Kelvin and is highest at the center of the weld, and decreases along the width of the weld. Even at the edge of the weld the temperature often exceeds the melting temperature of the wear-reducing material, e.g., tungsten carbide. Tungsten carbide introduced into a weld puddle (plasma envelope) often melts. The fine tungsten carbide goes into solution. The large carbides decrease in size from the melting of their outer surface. The resulting weld, i.e. tool joint surfacing, often contains only discrete tungsten carbide particles of a large size. These particles can become aggressive grinding elements when they contact steel components, i.e., the weld casing and casing lining a wellbore through which the tool joint will pass. Often attempts to counter the grinding effect, if smaller carbide particles are introduced into the weld, still results in an undesirably high percentage of melted tungsten carbide. Carbides that remain are often spaced-apart at a distance greater than the diameter of the pieces of abrading material (foreign earthen material particles that eat away the matrix that holds the carbides) that cause the carbides to fall out of the matrix affording little wear resistance.
In certain prior art processes as described above, an electric arc penetrates and dilutes the substrate, the material of the tool joint being surfaced. Even using very closely controlled parameters with current prior art processes, it is difficult to achieve a weld with less than 15% dilution. The disadvantage of this dilution is that a chosen material or alloy is diluted and cannot perform to engineered expectations.
There has long been a need for an improved method of application of wear-reducing materials that is effective in applying a range of appropriate materials to tool joints in order to increase service life. There has long been a need for such methods in which: carbides are applied without subjecting them to the intense heat of an electric arc, which dissolves the carbides; a deposit of alloy with minimal additional alloying of the tool joint's base material is deposited; such methods in which intense heat does not alter an alloy's or a material's desired chemistry; such methods in which fine carbides are not melted by intense heat; and such methods that lay down a re-producible hardfacing. There has long been a need for such methods that produce a metallurgical bond between a base material of a tool joint and wear-reducing or hardfacing material applied thereto and such methods which allow the base material of a tool joint to remain unchanged.